Turbine Engine Shaft Coating

ABSTRACT

A coated steel substrate has a steel substrate having a surface. A coating layer is atop the surface. The coating layer includes: aluminum activated by indium; and a ceramic binder. The coating also may comprise of multiple layers with different properties to facilitate the galvanic protection capability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 17/021,304,filed Sep. 15, 2020, and entitled “Turbine Engine Shaft Coating” whichclaims benefit of U.S. Patent Application No. 62/903,144, filed Sep. 20,2019, and entitled “Turbine Engine Shaft Coating”, the disclosures ofwhich are incorporated by reference herein in their entireties as if setforth at length.

BACKGROUND

The disclosure relates to gas turbine engines. More particularly, thedisclosure relates to coated steel shafts in gas turbine engines.

Gas turbine engines (used in propulsion and power applications andbroadly inclusive of turbojets, turboprops, turbofans, turboshafts,industrial gas turbines, and the like) use shafts in one or morelocations to transmit rotation. In the high pressure turbine (HPT)section a two-spool (or greater) engine, the temperature may be so highas to require use of a nickel-based superalloy. In the low pressureturbine (LPT) section of a two-spool (or greater) engine, temperaturesmay be low enough to allow use of steel to reduce cost. Exemplary shaftmaterial is a low-alloy, high strength, steel. The low alloy steelscontain minor non-ferrous elements and have yield strengths greater than275 MPa. Examples include AISI 4130, AISI 4340, AISI H-11 and the like.

High strength alloys are generally prone to atmospheric corrosion andrequire protective treatments such as coatings. However, elevatedoperating temperatures in an engine environment greatly limit thechoices of corrosion protection coatings. Specifically, organiccorrosion inhibitors would be compromised by thermal decomposition.Although inorganic corrosion inhibitors such as chromate have been foundto be effective in retarding metal corrosion, chromate is a carcinogenand is being phased out in industries. In addition, due to the inabilityto form passivation layer, the high strength steel alloys rely uponsacrificial coatings to impart corrosion resistance.

For elevated temperature applications, an aluminum-ceramic coating is anattractive option. An exemplary coating is aluminum powder in alkalimetal silicate (e.g., sodium silicate). See, U.S. Pat. No. 9,739,169, ofKlotz, et al., Aug. 22, 2017, entitled “Formation of corrosion-resistantcoating” (hereafter the '169 patent). Such coatings may be applied asaqueous slurries and may include additives such as wetting agents,organic solvents, and the like. The sacrificial Al-ceramic coatings arequalified mostly in accelerated salt fog tests such as ASTM B-117wherein high concentration chloride promotes and accelerates corrosion.Such a corrosive environment is considered representative of onlycoastal applications at ambient temperatures.

SUMMARY

One aspect of the disclosure involves a coated steel substratecomprising: a steel substrate having a surface; and a coating layer atopthe surface. The coating layer comprises: aluminum activated by indium;and a ceramic binder.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the coating layer comprising5% to 40% porosity.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the coating layer comprising,by weight: 0.5% to 40% ceramic binder including up to 2% impurities andadditives; and 60% to 97.5% aluminum alloy.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the aluminum alloy being atleast 90% aluminum by weight.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the aluminum alloy being 0.01%to 3.0% indium by weight.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the coating layer comprisingzinc.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the ceramic binder comprisingan alkali metal silicate.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the aluminum activated byindium comprising an Al—In alloy.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the aluminum activated byindium comprising: aluminum or an aluminum alloy; and indium oxide.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include an under-layer between thesurface and the coating layer and comprising: a ceramic binder andaluminum alloy in a naturally passivated state relative to the aluminumalloy of the coating layer.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the under-layer having a lowerindium content than the coating layer; and the under-layer and thecoating layer being each at least 20 micrometers thick.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include: the under-layer havingporosity of 7% to 25%; the coating layer having porosity of 15% to 38%;and the coating layer being more porous than the under-layer.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include a gas turbine engine (or otherturbomachine) including the coated steel substrate as a shaft.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include a method for manufacturing thecoated steel substrate, the method comprising: applying the aluminum andone or more precursors of the ceramic binder in one or more slurries;and heating to vaporize a carrier of the one or more slurries.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the aluminum being as at leastone of an Al—In or an Al—Zn—In.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the one or more precursorscomprising glass formers comprising one or more of Na₂O, K₂O, andsilica.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the one or more slurriescomprising indium oxide.

Another aspect of the disclosure is a coated steel substrate comprisinga steel substrate having a surface. A coating is atop the surface. Thecoating comprises: a first layer comprising aluminum and a ceramicbinder; and a second layer comprising a ceramic binder and at least 78%by weight aluminum activated by indium, the first layer between thesubstrate and the second layer.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the ceramic binder in thefirst ceramic coating layer and the ceramic binder in the second ceramiccoating layer each comprising an alkali metal silicate.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include: the first layer having alower indium content than the second layer; and the first layer and thesecond layer being each at least 10 micrometers thick.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include: the aluminum of the firstlayer being in a naturally passivated state and less active than thealuminum of the second layer.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the coated steel substratebeing a turbomachine shaft.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a coated substrate with afirst coating system.

FIG. 1A is an enlarged view of the coating of FIG. 1 .

FIG. 1B is a further enlarged view of a portion of the coating of FIG.1A.

FIG. 2 is a schematic cross-sectional view of a coated substrate with asecond coating system.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The sacrificial properties of the conventional aluminum-ceramic coatingappear to be largely defeated in atmosphere containing aggressive airpollutants such as sulfur dioxide and particulate matter. One possiblecause of this failure is the partial passivation of the aluminum powderin the presence of sulfuric acid or related sulfates. See, LOTO,CLEOPHAS & POPOOLA, PATRICIA, “Effect of anode and size variations onthe cathodic protection of mild steel in sea water and sulphuric acid”,International Journal of the Physical Sciences, Jun. 18, 2011, pp.2861-2868, Vol. 6(12), Academic Journals, Lagos, Nigeria. In addition,solid particles and deposits can physically hamper the mass transportcritical to enable the underlying galvanic protection mechanisms.

To improve protection relative to the use of aluminum powder of the '169patent, alternative alloys may include zinc and/or indium. Exemplaryalloys are Al—In, Al—Zn, and Al—Zn—In. As with the '169 patent, thecoating is made with a slurry containing the alloy powder and ceramicprecursors and heat treated to form a ceramic binding network.Accordingly, FIG. 1 shows an article 20 having a metallic substrate 22.As discussed above, exemplary substrates are steel and form shafts of agas turbine engines or other turbomachines. The article has a coatingsystem (coating) 24 atop a surface 26 of the substrate. The coating 24itself has a surface (outer surface) 28. The exemplary coating 24 is acorrosion protection coating for protecting the substrate 22 fromcorrosion. Exemplary coating thickness T is 10 micrometers to 100micrometers, more particularly 25 micrometers to 75 micrometers.

As is discussed further below, the coating 24 comprises the activatedaluminum alloy 40 (FIG. 1A) in particle form and a ceramic binder 42forming an interconnected matrix holding the aluminum particles.Interconnected porosity 43 (FIG. 1B) allows air to permeate through thecoating to reach the substrate. This is effectively a pre-use conditionafter the coating has been cured (chemically cured and/or solventevaporation) but before any in use reaction with environmental sources(e.g., air, pollutants, combustion products, and the like) or with thesubstrate.

In some embodiments, the structure may be essentially a gas diffusionelectrode where the solid electrode (Al alloy particles 40) is wrappedby the binder 42 and the space (porosity 43) created by the alloy-binderparticle agglomerates serve as gas passages. The gas passages areessentially connected voids or air pockets. These pores may besufficient in volume so that the electrode cannot be fully flooded,which would otherwise impede oxygen transport to the substrate.

A key activator is indium. Indium, either in alloyed form or free ionsin electrolytes, has been shown to activate aluminum that otherwisewould be passivated, particularly in a sulfuric acid environment. See,C. B. BRESLIN, L. P. FRIERY, AND W. M. CARROLL, “Influence of ImpurityElements on Electrochemical Activity of Aluminum Activated by Indium”,CORROSION, November, 1993, pp. 895-902, Vol. 49(11), NACE International,Houston, Tex.

The Al alloy powder or flakes used in the coating typically range from 1micrometer to 20 micrometers. For example, a D50 size may be 1micrometer to 20 micrometers, more particularly 2 micrometers to 10micrometers or 3 micrometer to 8 micrometers. To allow high temperatureoperation, the Al alloy and any other main coating constituents arebonded by a ceramic binder. The precursors of the binders includetypically a glass-former such as SiO₂ and/or a phosphate and an alkalielement (e.g., in the form of oxides such as in the '169 patent (e.g.,Na₂O, K₂O)). The two components' precursors are kept in separateslurries and mixed (e.g., with the Al alloy powder which, along withother additives, may be in its own slurry) prior to coating application.Stoichiometric amounts of the binder precursors may be selected toensure there is no excess amount of non-conductive inclusions in theresulting coating.

Exemplary final (pre-use) coating composition has 5% to 40% porosity,more narrowly 7% to 38% or 15% to 32%. Exemplary final coatingcomposition is, by weight, 60% to 97.5% the Al alloy (or at least 78% ora range of 85.0% to 96.0%), remainder essentially binder with minoroptional additives and impurities. Exemplary impurities are no greaterthan 1% by weight, preferably less than 0.50%. Exemplary additives areno greater than 1% by weight, preferably less than 0.50%.

For the Al—In alloy, exemplary weight percent composition is 0.01% to3.0% In, more narrowly 0.10% to 3.0%, 0.10% to 2.0%, or 0.40% to 1.5%.Al with intra-alloy impurities and alloyants, if any, is substantiallythe balance (e.g., 97% to 99.99%). Such exemplary impurities and/oralloyants within the alloy may aggregate to up to an exemplary 10% byweight. This may leave the alloy with at least 90% Al or at least 95% orat least 97% or up to that 99.99% depending on purity. Exemplaryindividual element concentration of the impurities or alloyants may beup to 5% or up to 3% or up to 2%.

For the Al—Zn—In alloy, exemplary weight percent composition is: 0.01%to 0.20% In, more narrowly 0.02% to 0.10%; and 0.5% to 5.0% Zn, morenarrowly 1.0% to 3.0%. Al with intra-alloy impurities and alloyants, ifany (in amounts noted above) is substantially the balance (e.g., 94.9%to 99.48% or 95.0% to 99.5%).

For the Al—Zn alloy, exemplary weight percent composition is 0.5% to5.0% Zn, more narrowly 1.0% to 4.0%, or 1.5% to 3.0%. Al withintra-alloy impurities and alloyants, if any (in amounts noted above) issubstantially the balance (e.g., 94.5% to 99.5% or 96.0% to 99.0% or97.0% to 99.0%).

More generically, for these and other variants/embodiments, the alloymay be at least 90% Al, more narrowly at least 95% or at least 97% or90% to 99.9%, 95% to 99.9% or 97% to 99.9%.

Alternatively to use of such alloy, a compound soluble in an acidicenvironment over a range of temperatures can be added to the coating,which will be liberated to activate the Al powder. For example, thiscompound may contain indium (e.g., indium oxide). In an embodiment, thismay involve adding a small amount of indium oxide to the slurry of the'169 patent. An exemplary amount is up to 8%, more particularly up to 5%or 0.5% to 8.0% or 1.5% to 5.0%.

Further embodiments include multiple layers of differing properties,including compositions and/or porosity. In one group of embodiments, theporosity increases progressively away from the substrate even ifas-applied chemical composition is otherwise the same. This could alsobe a continuous gradation of porosity without discrete layers. Anexemplary delta of porosity is 5.0% to 20.0% or 5.0% to 15.0%

As an example of a chemical variation, FIG. 2 shows an article 100 witha two-layer (bi-layer) coating system 102 atop the substrate surface 26.The coating system 102 has a first layer (an under-layer or inner layer)104 atop the substrate and a second layer 106 (top layer or upper layeror outer layer) atop the first layer. One group of these embodimentsinvolves a bi-layer coating system where the under-layer 104 is ofconventional Al-ceramic coating (e.g., of the '169 patent) and isdeposited on the substrate followed by the aforementioned active layermaterial for the outer layer 106. The first layer composition may havesimilar aluminum and ceramic contents to those given above with lesser,if any, In (likely at most 50% or at most 10% the In of the secondlayer). Exemplary first layer thickness T₁ is 10 micrometers to 40micrometers, more particularly 20 micrometers to 38 micrometers.Exemplary second layer thickness T₂ is 10 micrometers to 40 micrometers,more particularly 20 micrometers to 38 micrometers.

Embodiments of the bi-layer coating 102 may provide advantageousprotection to the substrate in both highly active chloride environments(where the less active under-layer 104 is activated due to lower ohmicresistance) and in a more passive sulfuric acid environment (where thetop layer 106 becomes active). Thus, various embodiments of the bi-layercoating 102 will enhance the robustness of the coating in a range ofcoastal and industrial environments relative to a baseline coating ofthe '169 patent.

The bi-layer (or more) coating 102 with different Al pigments allows oneindividual layer to be more active than another in a given environmentto address operation in multiple environments. In one group ofembodiments, the under-layer 104 is favored due to less ohmic drop whenthe galvanic cell is activated and contains a less active Al pigment;whereas the top layer 106 comprises a more active Al alloy pigment(e.g., Al—Zn—In, Al—In alloys) or a mixture of Al alloy and anactivator.

After curing, the coating needs to possess connectivity for both thepigments and pores. The electrical connectivity and porosity will enableelectrochemical oxygen reduction coupled with aluminum dissolution tomake possible the galvanic protection of the substrate. Thus, furtherembodiments may have a graded porosity in the bi-layer coating. Forexample, the top layer 106 may have higher porosity to alloy oxygentransport even when there may be deposits from air pollutants that tendto block the gas passages. The under layer 104 may have lower porosityin order to maintain sufficient throwing power of the localized galvaniccouple for protection whereas the top layer 106 supports mass transportwhen surface contaminants tend to block fine pores during service. Bothsolid metal content and pores will need to exceed their percolationthresholds, which can vary based on the shape of the solid particles.

In further variations on the foregoing embodiments, stabilizers (e.g.,vanadate, molybdate, and/or cerium citrate stabilizers) can be added tothe slurry formulation to regulate the spontaneous reactions of Al andAl alloy pigments during storage and coating application. Thestabilizer(s) retard reactions of active Al pigment with slurrycompounds to ensure shelf lifetime of the mixture and extended time forcoating application. Exemplary concentrations of the inhibitors rangefrom 0.0010 M to 0.20 M, more narrowly 0.050 M to 0.20 M, in thecombined/final pre-application slurry.

In addition, to the aforementioned high strength, low alloy steel shaft,other materials and articles may benefit from the coating. High strengthsteels such as UNS K92580/AMS 6532 (ultra-high strength martensiticsteel) with higher Ni contents can also particularly benefit from thecoating for operation at elevated temperatures. Exemplary componentsbased on those alloys in aero engines include: high pressure compressorrotor hub; low pressure compressor rotor drum; turbine shaft coupling;high pressure turbine hub sleeve and bearing seal front seat. Auxiliarypower units (APU) components such as ring gears for aircraft alsoutilize the high strength steels that need to be protected. The coatingis suitable for corrosion protection of those parts that are prone tocorrosion but with less concerns of wear resistance.

The use of “first”, “second”, and the like in the following claims isfor differentiation within the claim only and does not necessarilyindicate relative or absolute importance or temporal order. Similarly,the identification in a claim of one element as “first” (or the like)does not preclude such “first” element from identifying an element thatis referred to as “second” (or the like) in another claim or in thedescription.

Where a measure is given in English units followed by a parentheticalcontaining SI or other units, the parenthetical's units are a conversionand should not imply a degree of precision not found in the Englishunits.

One or more embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, whenapplied to an existing baseline configuration and/or composition,details of such baseline may influence details of particularimplementations. Accordingly, other embodiments are within the scope ofthe following claims.

1. A method for manufacturing a coated steel substrate, the coated steelsubstrate comprising: a steel substrate having a surface; and a coatinglayer atop the surface, wherein the coating layer comprises: aluminumactivated by indium; and a ceramic binder, the method comprising:applying the aluminum and one or more precursors of the ceramic binderin one or more slurries; and heating to vaporize a carrier of the one ormore slurries.
 2. The method of claim 1 wherein the coating layercomprises: 5% to 40% porosity.
 3. The method of claim 1 wherein thecoating layer comprises, by weight: 0.5% to 40% ceramic binder includingup to 2% impurities and additives; and 60% to 97.5% aluminum alloy. 4.The method of claim 3 wherein: the aluminum alloy is at least 90%aluminum by weight.
 5. The method of claim 3 wherein: the aluminum alloyis 0.01% to 3.0% indium by weight.
 6. The method of claim 1 wherein thecoating layer comprises: zinc.
 7. The method of claim 1 wherein: theceramic binder comprises an alkali metal silicate.
 8. The method ofclaim 1 wherein: the aluminum activated by indium comprises an Al—Inalloy.
 9. The method of claim 1 wherein the aluminum activated by indiumcomprises: aluminum or an aluminum alloy; and indium oxide.
 10. Themethod of claim 1 wherein the coated steel substrate further comprises:an under-layer between the surface and the coating layer and comprising:a ceramic binder and aluminum alloy in a naturally passivated staterelative to the aluminum alloy of the coating layer.
 11. The method ofclaim 10 wherein: the under-layer has a lower indium content than thecoating layer; and the under-layer and the coating layer are each atleast 20 micrometers thick.
 12. The method of claim 10 wherein: theunder-layer has porosity of 7% to 25%; the coating layer has porosity of15% to 38%; and the coating layer is more porous than the under-layer.13. (canceled)
 14. (canceled)
 15. The method of claim 1 wherein: thealuminum is as at least one of an Al—In or an Al—Zn—In.
 16. The methodof claim 1 wherein: the one or more precursors comprise glass formerscomprising one or more of Na₂O, K₂O, and silica.
 17. The method of claim1 wherein: the one or more slurries comprise indium oxide. 18.(canceled)
 19. (canceled)
 20. (canceled)
 21. (canceled)
 22. (canceled)